Relaxation Dynamics of Large and Anisotropic Molecules in a Weakly Polar Matrix

The dynamics of large and anisotropic molecules embedded in glass-forming liquids present complex challenges due to the interplay between molecular shape, host-guest interactions, and macroscopic viscosity. In this work, we investigate fluorophenylene isomers (ortho-F and para-F) dissolved in the ortho-terphenyl (OTP) matrix, combining broadband dielectric spectroscopy, differential scanning calorimetry, and rheology to disentangle host and probe dynamics. While calorimetry reveals a single glass transition dominated by OTP, dielectric measurements resolve two distinct α-relaxation processes: a fast process corresponding to OTP structural relaxation and a slower process associated with probe relaxation, i.e., reorientation of large and anisotropic molecules in a weakly polar matrix. Furthermore, the α-relaxation peak of the guest molecules becomes noticeably narrower upon dilution in OTP. We attribute this behavior to the dynamic homogenization of the local environment arising from the large size mismatch between the guest and the host molecules. Mechanical relaxation times derived from viscosity match the fast dielectric α-process, indicating that OTP exclusively governs the viscous response. Interestingly, the para-F/OTP mixture shows ideal Debye-Stokes-Einstein (DSE) coupling, whereas ortho-F/OTP exhibits systematic decoupling, reflecting the importance of probe dipole orientation and anisotropy.